Could gut bacteria help make you more anxious?

Gut brain connectionStress during childhood can affect the make-up of our gut bacteria and this, in turn, could influence our mental health as adults, a new mouse study suggests.

The research found transplanting gut bacteria from mice stressed when they were young could encourage anxiety-like behaviour in some recipients.

The researchers say, among other things, the study raises questions about how faecal transplants in humans might affect our brains as well as our guts.

"Many of my patients are those who have functional bowel disorders - whether it's irritable bowel syndrome or functional dyspepsia - and a significant proportion of them they have some psychiatric comorbidities [such as] depression or anxiety," co-author and gastroenterologist Dr Premysl Bercik of McMaster University in Canada.

The new study, published today in Nature Communications, is part of ongoing research into how the gut influences behaviour.

Shy and daring mice

Previous studies in mice have shown that it's possible to change their behaviour by treating them with antibiotics, which changes the bacterial population of the gut, or by colonising their guts with bacteria taken from different 'personalities' of mice.

"If you colonise the shy mouse, which is germ-free [bred to have no gut bacteria], with the microbiota from the daring mouse, you change their behaviour and you make them more daring," Bercik says.

In this study, researchers were trying to tease out the role that gut bacteria play in the adult consequences of early-life stress.

They found that stress in early life - in this case, baby mice being separated from the mother for several hours a day in the first few weeks of life - changed the intestinal bacteria and led to anxiety-like behaviour in adulthood.

However, stress in early life did not affect adult behaviour in mice bred to be germ-free, and which therefore had no gut bacteria at all.

Such mice had high levels of the stress hormone corticosterone but they behaved normally, like the control mice, which had not been stressed when they were young, Bercik says.

If those same germ-free mice, who had been subjected to early-life stress, were then colonised with the gut bacteria from a mouse that had been stressed, only then did they show the abnormal behaviour.

"That means that bacteria are actually required for the induction of this abnormal anxiety or depression-like behaviour," says Bercik.

But a transplant from a stressed normal mouse by itself wasn't enough to induce behavioural change if the germ-free mice had not experienced early-life stress as a result of maternal separation.

"Thus, it is evident that the behavioural phenotype of this model of early-life stress reflects a convergence of microbial and host factors," the authors write in the journal.

Gut brain connection

Bercik says gut bacteria are known to produce neuro-active molecules, which could be one way that they are able to influence the brain.

He and colleagues analysed the metabolic activity of the gut bacteria present in the stressed mice and the findings suggested that this activity did change in response to stress.

The study could have implications for the growing practice of faecal transplants, which are most commonly used to treat gut problems associated with antibiotic use.

While there is currently concern about whether the donor has parasites or infectious diseases, Bercik his findings suggest there are other things to worry about.

"We should maybe think: 'Is this human donor really healthy from the mental point of view? Does he or she have any history of depression or anxiety?'," says Bercick.

"Because there is a chance that we might be also transferring some susceptibility or possibility of developing these kinds of disorders to the recipient."